Apparatus and methods for energizing a printhead are well known in the prior art. In the prior art when utilizing a power supply to energize a printhead, such as a dot matrix impact printhead, and more particularly when using an unregulated bi-level voltage drive, the power supply voltage fluctuates due to changing printhead duty cycle. As the duty cycle increases the voltage output of the power supply tends to decrease. The voltage change which can be in the range of 5 to 7 volts can reduce the coil current in the printhead actuators by as much as 25 percent. Reduction in coil current reduces the energy available to actuate the printhead and degrades the print quality.
In the known prior art, the printheads include a plurality of actuators, such as coils, which are periodically energized by pulses of power to actuate the printhead and effect printing. As the duty cycle of the actuators increases, the voltage available to the actuators from the power supply decreases and the coil current also decreases with a resulting decrease in available actuator energy. To offset the problem of decreasing voltage and current in the printhead actuators, the "time on" for the printhead actuators must be increased in order to maintain substantially constant peak current in the printhead actuators. The prior art attempts to solve this problem by utilizing a one-shot multivibrator to monitor the voltage pulses through the printhead and to lengthen the voltage pulses in response to a decreasing voltage. Thus, the prior art corrects the problem by monitoring the supply voltage and increasing the "time on" for the voltage pulse with a multi-vibrator wherein the "time on" is dependent upon the sensed supply voltage. In such prior art apparatus, expensive and space consuming circuitry must be utilized to monitor the supply voltage and control the pulse width from the multi-vibrator.
The present invention overcomes the disadvantage of the prior art by not directly sensing supply voltage drop. The present invention senses the duty cycle of the printhead and estimates the supply voltage drop in dependence upon the sensed duty cycle. It is assumed that the supply voltage drops as the duty cycle increases. The length of the pulses for energizing the actuators of the printhead is then modified in dependence upon the sensed duty cycle of the printhead. No monitoring of the supply voltage occurs in the present invention.